1. Field of the Invention
The present invention relates to a method for dispersing nanoparticles. It also relates to a method for producing nanoparticle thin films by utilizing the dispersed nanoparticles. More specifically, the present invention relates to a method for producing a nanoparticle thin film in which the surface of nanoparticles is modified to allow the nanoparticles to be charged. An electrostatic attractive force between the charged nanoparticles and a substrate and a repulsive force between the individual nanoparticles is controlled by a variation in pH to control the number density of the nanoparticles arranged on the substrate. In addition, a capillary force other than the electrostatic forces is introduced during the arrangement of the nanoparticles to form a high-density and uniform nanoparticle monolayer in a large area on the substrate.
2. Description of Related Art
Recently, research has been conducted to use nanoparticles in various industrial applications, such as memory devices with maximized memory capacity. Nanoparticles are also used in light-emitting devices utilizing the luminescence of nanoparticles due to quantum confinement effects. However, since nanoparticles have a strong tendency toward aggregation between the individual nanoparticles, they do not disperse easily. Various methods have been attempted to improve the dispersibility of nanoparticles without aggregation in media.
Certain techniques have been known to improve the dispersibility of nanoparticles by displacing materials coordinated to the surface of other nanoparticles. For example, sonication and filtration using a column or a filter have been employed to separate nanoparticle aggregates. However, when sonication is applied to separate nanoparticle aggregates for a long period of time, high ultrasonic energy used during the sonication results in the destruction of nanoparticles or causes the occurrence of defects of nanoparticles. If the reaction time is shortened in order to prevent the occurrence of defects, some reactants remain unreacted and the yield becomes low.
In addition, when aggregated particles are separated from each other using a column or a filter, a high water pressure has to be applied to the filter for a long time and the nanoparticles are adsorbed to the filter, resulting in a loss of the nanoparticles. Accordingly, these separation processes are unsuitable for mass production of nanoparticles without aggregation.
Langmuir-Blodgett (LB) technique wherein a film is formed at the interface between an aqueous solution and air is another technique for producing nanoparticle thin films. However, since this technique utilizes weak van der Waals interactions between individual particles or between particles and the substrate, the transfer ratio, which is defined as the percentage of the number of the particles transferred to the substrate, is usually never higher than 1. Accordingly, the Langmuir-Blodgett technique is not suitable for the formation of uniform monolayers in large areas. Electrostatic self assembly technique is also used for producing nanoparticle thin films. According to this technique, nanoparticles and a substrate oppositely charged to the nanoparticles are used to produce a thin film. However, this technique causes the aggregation of nanoparticles in its practical use, which results in the occurrence of defects and low degree of completeness.
Pyrolysis, laser ablation and chemical vapor deposition (CVD) techniques are also used to produce nanoparticle thin films. According to these techniques, nanoparticles are directly formed on a substrate through a vapor phase reaction of raw materials supplied in a gaseous state, followed by sequential deposition and growth to arrange the nanoparticles on the substrate. However, these techniques have problems in that a uniform monolayer having a density as high as 1011 nanoparticles/cm2 cannot be formed in a large area.
Thus, there is a need for a nanoparticle thin film that has a large surface area and is in the form of a uniform monolayer. It is also desirable for the nanoparticle thin film to have a high-density of nanoparticles.